You neglected to mention your OS. I'm guessing that will help the more knowledgeable members arrive at a diagnosis.
Are you sure you don't have a faulty RAM stick in there somewhere? Barring that, have you been experiencing power failures in your area lately? Forgive me for asking, but when you say you reformatted your system to factory do you mean you did a clean re-install of Windows?
At what stage of the boot process does your PC "hang"? Does it happen during post or while the OS is loading?
I any case, I would check the boot drive for errors and then maybe defragment the same. If that does not work then perhaps you should do a quick check for loose SATA cables and connections. Have you installed any new applications or drivers recently?
